I'm not much of a shiny expert, but from the dashboards I've been building, here is what I would do. I would create a reactiveValues variable called selectedData. Then in my observeEvent, I would pull the csv and update the selectedData. The reason why I chose to use reactiveValues in my app is that they can be stateful rather than just always reacting to some input, and I can call them in other areas of the app that requires the same information. As for using observeEvent, it's simple because you can then wrap your updateSelectizeInput within the observer.
shinyServer( function(input, output, session) { # init reactiveValues selectedData <- reactiveValues() observeEvent(input$selectData, { if (input$selectData == "Universities") { selectedData$orgdata <- read.csv("universities.csv") } else if (input$selectData == "Hospitals") { selectedData$orgdata <- read.csv("hospitals.csv") } updateSelectizeInput(session, "Organisations", choices = as.character(unique(selectedData$orgdata$name)), server = TRUE) }) ... # rest of your code })